I would summarize my opinion as :
0. Moving forward or changing lanes, Side mirrors and rear view mirror/camera work very well. I don't notice any rear visibility issues in these scenarios. Along with blind spot monitoring, I think pretty much all scenarios are covered.
1. Looking over the shoulder while backing out won't be too helpful. I don't know how is this different from other vans. Your 5:00 to 7:00 if sitting at the center of the clock looking at 12, is mostly blocked. The rear window is kind of small and high to spot something close behind tail lights.
2. I find myself mostly relying on backup camera, and also birds-eye view, on infotainment screen. I have yet to differentiate between various rear cameras, but the rear-view mirror camera can be helpful too. I have yet to back up only using the rear-view-mirror camera though.
3. This is where the rubber meets the road : IIRC, all trims have rear parking assist, and Rear Cross Traffic alert. Comparing backing out a small sedan and Sienna, additional risk of running in to something is very low unless you're not paying attention, or are in a super hurry. Key is to take your time and use all available cameras and features.
Caveats :
1. Snow and rain have tendencies get over all cameras and cloud or distort various views. I wish they put tiny air jets to clean them up. I don't know what other makers do so this might not be a Toyota only issue.
2. Infotainment screen is not as crisp, or the cameras are not very high resolutions. Sometimes, especially in snow, back-out views seems to get somewhat washed out as if the screen is too bright. I will have to play with screen brightness, but I doubt it is the only cause.
Folks with Reading-Only glasses : Keep them handy while working in/out of tight parking spots. You will need them to look at camera views!